1. Field of the Invention
The present invention relates to thin film forming techniques employed in the manufacture of a semiconductor device such as a VLSI. More particularly, the present invention relates to a thin film forming method and apparatus using chemical vapor deposition (CVD).
2. Description of the Related Art
In recent years, CVD is widely used in the process of manufacturing a semiconductor device. This CVD includes the following steps. A predetermined gas is fed to a reaction furnace in which a substrate is placed to chemically react the gas on the substrate surface, thereby depositing a thin film on the substrate. In plasma CVD and photo-assisted CVD among the CVD methods, since activation efficiency of a gas is increased by a plasma or a beam, a thin film can be deposited at a high rate even at a low temperature. For this reason, the plasma CVD and the photo-assisted CVD are effective in the manufacture of a semiconductor device. Therefore, the plasma CVD and the photo-assisted CVD are expected to be important in accordance with a decrease in temperature in the process of manufacturing a semiconductor device.
However, the these CVD methods have the following problems. That is, in plasma or photo-assisted CVD, a high-frequency wave or a beam is irradiated on a gas. For this reason, atoms consisting of the gas are excited. A primary decomposition reaction of the gas occurs until the gas reaches the substrate. As a result, many types of intermediates produced by the primary decomposition reaction are mixed in a deposition film. Some of the intermediates cause degradation of the density, chemical resistance, and hardness of the thin film formed on the substrate. Therefore, performance and reliability of the semiconductor device are degraded.
In the normal CVD, the primary decomposition reaction may be performed by heat. Therefore, these problems are applied to not only the plasma and photo-assisted CVD methods but all other CVD methods.